Mismatch, or imbalance, between an in-phase (I) and a quadrature (Q) signal path in a quadrature receiver circuit limits the achievable image attenuation, which results in a distorted signal. Likewise, imbalance between an I and a Q signal path in a quadrature transmitter circuit also poses a limitation on the achievable image attenuation.
Various techniques have been developed for compensation of such imbalance in order to mitigate the effects of the imbalance and provide improved image attenuation. For example, the article L. Antilla et al, “Circularity-based I/Q imbalance compensation in wideband direct-conversion receivers”, IEEE Transactions on Vehicular Technology, vol. 57, no. 4, pp. 2099-2113, July 2008, discloses compensation of I/Q imbalance in quadrature receivers. In the following, this article is referred to as the Antilla receiver paper. Furthermore, the article L. Antilla et al, “Frequency-selective I/Q mismatch calibration of wideband direct-conversion transmitters”, IEEE Transactions on Circuits and Systems—II: Express Briefs, vol. 55, no. 4, pp. 359-363, April 2008, discloses compensation of I/Q imbalance in quadrature transmitters. In the following, this article is referred to as the Antilla transmitter paper
FIG. 1 illustrates a block diagram of a basic compensation circuit 1 utilized in both of the articles mentioned above. A complex-valued signal o1(n) is an input signal to the compensation circuit 1 and another complex-valued signal o2(n) is output from the compensation circuit 1. In the case of a receiver circuit, o1(n) is a signal, representing received data, having unbalanced I and Q components that is processed by the compensation circuit 1 to generate the signal o2(n) with (ideally) restored balance between the I and Q components. In the case of a transmitter circuit, o1(n) is a signal, representing data to be transmitted, having balanced I and Q components, which is processed by the compensation circuit 1 to generate the signal o2(n) with imbalance between the I and Q components that compensates for the imbalance in the I and Q signal paths of the transmitter such that the transmitted radio-frequency (RF) signal (ideally) has balanced I and Q components. In either case, the signal o1(n) is input to a block 2 that generates the complex conjugate o*1(n) of o1(n), which is filtered by a filter 3 having the frequency response W(ejω). The output signal of the filter 3 is added to the signal o1(n) in the adder unit 4 to generate the signal o2(n).
It is desirable to provide efficient compensation of imbalance between I and Q signal paths of a quadrature receiver or a quadrature transmitter at a relatively low computational complexity, e.g. in order provide a relatively small overhead in terms of required circuit area and/or power consumption for performing the compensation.